1. Field of the Invention
The invention is directed to an improved fuel injection system for an internal combustion engine.
2. Description of the Prior Art
One fuel injection system, known from German Patent Disclosure DE 198 35 494, has a fuel injection valve, with an injection valve member by which at least one injection opening is controlled. The injection valve member is urged in an opening direction counter to a closing force by the pressure prevailing in a pressure chamber of the fuel injection valve. The pressure prevailing in the pressure chamber is generated by a pump piston of a high-pressure fuel pump, which piston is driven in a reciprocating motion by a cam. A control valve actuated by a piezoelectric actuator is provided, which is triggered by an electric control unit and by which a communication of the pressure chamber with a relief chamber is controlled. The control valve has a control valve member, upon which the actuator acts via a hydraulic coupler in order to move it between an opened position and a closed position. When the control valve member is in its closed position, the pressure chamber is disconnected from the relief chamber, and high pressure for a fuel injection can build up in it. The instant the closing position of the control valve member is reached is thus of great significance for controlling the fuel injection. Since the control valve member is disconnected from the coupler by the actuator, however, no information about this instant is available. Furthermore, proper function of the control valve also requires complete filling of the coupler.
The fuel injection system of the invention has the advantage over the prior art that the function of the control valve can be monitored. During the charging phase of the piezoelectric actuator, a pressure builds up in the hydraulic coupler that also has a feedback effect on the actuator after the charging process has been completed, and in the actuator, once the voltage supply has been cut off, this pressure generates a piezoelectric voltage that is characteristic for the pressure conditions in the coupler and accordingly also for the conversion of the trigger voltage into the stroke of the control valve member. Thus the voltage between the electrical terminals of the piezoelectric actuator can be used, without requiring a further sensor, as a measured parameter for the valve behavior.
Various advantageous features and refinements of the fuel injection system of the invention are disclosed. The measured voltage can be used to ascertain the instant when the closing position of the control valve member is reached. The course of the voltage can be monitored, for the occurrence of a minimum point in the curve course. Once the charging of the actuator has been completed, the pressure in the coupler initially drops over time, since the actuator, after the completion of the charging process, has nearly reached its complete stroke, while the control valve member at this instant is still moving towards its closing position, and the coupler is thus depressurized. However, as soon as the control valve member has reached the closing position and is moving back toward its opening position because of its recoil, a compression of the medium located in the coupler occurs, which makes itself felt in an increase in the terminal voltage. The minimum point occurring in the course of the voltage thus identifies the instant at which the control valve member has reached its closing position. Alternatively, the derivation over time of the terminal voltage can also be formed and monitored for a zero crossover. The zero crossover of the voltage signal derived over time thus likewise identifies the minimum point in the course over time of the terminal voltage and thus the attainment of the closing position by the control valve member. In a further advantageous feature, a correction value for a control parameter of the control unit is derived from the ascertained instant when the closing position of the control valve member is reached. In particular, a correction value for the triggering voltage of the actuator and/or for the instant of triggering and/or for the duration of the charging process can be furnished. The voltage can also be used to detect the fill state of the coupler, since the pressure course in the coupler is dependent on the fill state and thus likewise has feedback effects on the voltage. The test triggering provided makes it possible to monitor the function of the control valve before a fuel injection, and correction values that can be used in the ensuing fuel injection can be ascertained. The time interval between the ascertainment of the correction value and the triggering of the control valve for the fuel injection is thus very brief, and so the fuel injection can be effected with high precision.